1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) demodulating apparatus and method thereof that demodulates an OFDM signal.
2. Description of the Related Art
In recent years, a modulating method called Orthogonal Frequency Division Multiplexing method (called OFDM method hereinafter) has been proposed for modulating digital data. In OFDM method, data is assigned and is digitally modulated to the amplitude and phase of each of many orthogonal sub-carriers within a transmission band by PSK (Phase Shift Keying) or a QAM (Quadrature Amplitude Modulation).
OFDM method has been widely considered to apply to terrestrial digital broadcasting, which is susceptible to multipath interference.
A send signal by OFDM method is transmitted in symbols called OFDM symbols as shown in FIG. 14. The OFDM symbol includes a valid symbol and a guard interval. The valid symbol is a signal period when IFFT (Inverse Fast Fourier Transform) is performed upon transmission. The guard interval is a copy of the waveform of a part of the second half of the valid symbol. The guard inertial is provided in the first half of the OFDM symbol and may have a time length of ¼ or ⅛ of that of the valid symbol, for example.
In an OFDM receiving apparatus that receives the OFDM signal, FFT (Fast Fourier Transform) calculation is performed thereon by an FFT calculating circuit, whereby the received OFDM signal is demodulated. From the OFDM symbol including a valid symbol and a guard interval, the OFDM receiving apparatus detects the boundary position of the OFDM symbol, defines a calculation range (called FFT window) of a length equal to that of the valid symbol from the detected symbol boundary position, identifies the data of the part defined by the FFT window from the OFDM symbol and performs FFT calculation thereon.
By the way, terrestrial broadcasting is a transmission path in a multipath atmosphere. In other words, terrestrial broadcasting is susceptible to the interference by delay waves due to the surroundings such as the surrounding geography and buildings at the receiving point, and the signal received by an OFDM receiving apparatus becomes a composite wave resulting from the combination of multiple delay waves.
In the transmission path in a multipath atmosphere, multiple symbol boundaries exist because of the existence of multiple paths. In this case, the inter-symbol interference may be generally avoided by defining the position of an FFT window based on the position of the symbol boundary of the first reached path.
Here, a method of defining the position of an FFT window, which determines the position of FFT calculation, will be described (see JP-A-2002-368717 and JP-A-2001-292125).
A first method of defining an FFT window delays an OFDM signal before FFT calculation is performed thereon, obtains the correlation between the waveform of the guard interval part and the waveform of the second half of the OFDM symbol (that is, the signal waveform from which the guard interval is copied), and calculates the boundary of the OFDM symbol. In this method, the time exhibiting the peak value of the autocorrelation function is the boundary of the OFDM symbol of each path.
A second method applies scattered pilot signals (SP signals) with a specific level and specific phase, which are scattered at specific positions in an OFDM symbol. The method estimates the transmission path characteristic of all OFDM symbols by extracting the SP signal from an OFDM signal, removing the modulated component therefrom and performing interpolation thereon by using a time direction interpolation filter. Then, a delay profile exhibiting the signal strength of each path is created by performing IFFT calculation on the estimated transmission path characteristic, and the boundary of the OFDM symbol is obtained based on the first reached path. Notably, the time direction interpolation of SP signals is for increasing the range of the detection of a delay profile by decreasing the intervals of SP signals artificially.
In general, in defining an FFT window, the first method causes a rough trigger to an FFT window at the beginning of receipt, and, after a predetermined period of time of the continuation of the first method, the second method causes the trigger to an FFT window. In other words, the establishment of rough synchronization is followed by more accurate symbol synchronization, resulting in a stable reproduction state.
A third method of defining an FFT window is also known that extracts the waveform of a guard interval part from an OFDM signal before FFT calculation is performed thereon, obtains the coherence between the extracted waveform and the waveform of the second half of the OFDM symbol and thereby obtains the boundary of the OFDM symbol. This method creates a delay profile exhibiting the signal strength of each path by obtaining the coherence and obtains the boundary of the OFDM symbol based on the first reached path.
Furthermore, in recent years, a method including a combination of the second and third methods has been proposed (see JP-A-2004-153831). The method allows the elimination of a false path by combining the delay profiles created by the two methods even when a false path due to noise exists in the delay profiles.
The second method may require the agreement between the interpolation value and the real transmission path characteristic since the SP signals are interpolated in the time direction. However, in an atmosphere in motion, the transmission path characteristic fluctuates at a high Doppler frequency, and a wrong delay profile might be created when the time-direction interpolation processing does not satisfy the sampling theorem.
FIGS. 15A and 15B show examples of cases in which the time-dependent fluctuation of a transmission path characteristic satisfies and does not satisfy a sampling theorem, respectively. As shown in FIG. 15A, when the time-dependent fluctuation of a transmission path characteristic satisfies the sampling theorem, the interpolation values (indicated by circles in white) of the SP signals (indicated by circles in black) agree with the real transmission path characteristic. On the other hand, as shown in FIG. 15B, when the time-dependent fluctuation of the transmission path does not satisfy the sampling theorem, the interpolation value (circles in white) of the SP signals (circles in black) do not agree with the real transmission path characteristic, resulting in the wrong estimation of the transmission path characteristic as indicated by the shown broken line.
FIG. 16 shows a delay profile resulting from the IFFT calculation performed when the time-dependent fluctuation of the transmission path characteristic does not satisfy the sampling theorem. As shown in FIG. 16, when the time-dependent fluctuation of the transmission path characteristic does not satisfy the sampling theorem, some time-direction interpolation may cause a pseudo path, which does not really exist, in addition to the real path that really exists. The pseudo path occurs regularly at the position of a normalized frequency of π/2, π or 3π/2 from the position of the real path.
Since a delay profile includes a pseudo path when time-direction interpolation is performed on SP signals, the definition of an FFT window may require the determination of either real path or pseudo path in order not to obtain the boundary of an OFDM symbol based on a pseudo path.
While a false path due to noise occurs at a random position, the pseudo path continuously occurs at a same position. Thus, when the position of the pseudo path in the delay profile created by the second method agrees with the position of the false path in the delay profile created by the third method, the pseudo path may be misdetermined as a real path even though the delay profiles of both are applied. As a result, the precision of the estimation of the delay profiles decreases.
Furthermore, in an atmosphere in motion, the precision of the estimation of delay profiles also decreases when the SNR (Signal to Noise Ratio) is deteriorated by a factor such as a fall of signal power.